I use a fan that is for cooling salt water canopy lights. It is 4.75" and comes ready to plug in the wall but only operates at 12v. They have a smaller size also depending how big your incubator is.

I don't have a link right now but it is coralife cooling fan kit. Comes in a blue box and the best part is it was about half the cost of a Radio Shack set up and that is for the larger fan. If anybody is interested in this I will find a link for it tomorrow or I mean later today.